Plot hole: When Margaret has to collect the $50,000, she goes around to various banks trying to get a loan on their house, which she can't get because her husband, co-owner of the house, isn't around. Why couldn't she just have taken the papers, said she was going to take them home to her husband, forged his signature, and brought them back? You can't exactly say she has either moral problems or isn't devious enough, since we've already seen her stuff a body into a dinghy, toss it into the lake, and come back to her house looking fairly normal.
Plot hole: At the end, if Margaret had followed Alek's plan and waited in the driveway after he killed Nagel, the police would have found Alek dead in the car crash with the video, which they would certainly have played, leading to not only Beau becoming a suspect , but also revealing his homosexuality - two issues they were blackmailing her for. Alek sacrificing his life would be in vain, except, perhaps for his own redemption.
Continuity mistake: In the car crash scene, Alec is in the car upside down, but when we see a shot of him we see blood going from his head to his chin which just wouldn't happen if you were upside down.